1. Technical Field
The invention relates to measurement of frequency and, more particularly, to devices capable of detecting very small changes in frequency, and signal generation circuits and signal generation methods that can be used for the devices.
2. Related Art
As frequency measurement systems, a direct count system and a reciprocal system are known. According to the direct count system, pulses passing within a predetermined gate time are counted (see, for example, Japanese laid-open patent application JP-A-2001-119291). According to the reciprocal system, a pulse period is accurately measured, and a frequency is obtained from its time reciprocal (see, for example, Japanese laid-open patent application JP-A-5-172861). The direct count system may be realized with a relatively small-scale circuit, but a long gate time may be needed to obtain a higher frequency resolving power (for example, a gate time needed for obtaining a resolving power of 0.1 Hz is 10 seconds). The reciprocal system can overcome this short-coming, but its circuit for accurately measuring pulse intervals becomes larger in scale, compared to that of the direct count system.
When a very small change in mass is to be detected, for example, when the presence or absence of odor substance adhered to a sensor wants to be detected, a method using changes in the frequency of the sensor caused by adhesion of substance may be utilized. For example, by using a QCM (Quartz Crystal Microbalance) method using a quartz oscillator as the sensor, a small change in mass on the surface of the oscillator can be converted to a frequency change. By using the oscillators and providing materials to which odor compositions adhere on the oscillators' surfaces, various kinds of odor sensors can be formed. Odor compositions may be each composed of a unit substance or multiple substances. As sample gas is supplied to the odor sensor, its odor compositions adhere to the sensor and change the mass at the surfaces of the oscillators, whereby the frequency of the oscillator changes, A single sensor or plural kinds of sensors may be prepared to observe the change, whereby the presence of specified odor composition can be detected.
In order to detect a frequency change of each of the odor sensors, a counter and a signal processing circuit for detecting the frequency change need to be added to each sensor output. Furthermore, although the frequency (for example, 30 MHz) of a quartz oscillator changes by adhered substance, the change is only on the order of several Hz to several hundreds Hz, and the change could be less than 1 Hz. As described above, the direct count system has a low frequency resolving power, and thus needs to take a substantially long gate time in order to improve its frequency resolving power. As errors that could occur at the time of measurement, plus/minus 1 count errors and errors caused by jitter at trigger level could occur, and in addition, errors that originate from the oscillation stability of the quartz oscillators would be superposed on the foregoing errors. The use of reciprocal system counters may supplement for the deficiencies described above, but would not be suitable for a sensor array that is equipped with many sensors, because the circuit scale of each of the counters becomes large.